Conventional piezoelectric ceramic hydrophones have employed relatively incompressible dense materials such as lead zirconate titanate (PZT). Because the hydrostatic sensitivity of such devices is only a fraction of their uniaxial sensitivity, randomly shaped sized and distributed pores have been provided in the ceramic material to enhance its compressibility and thereby the hydrostatic sensitivity. However, such hydrophones exhibit a number of shortcomings. The random pore arrangements tend to weaken the ceramic structure and the devices are susceptible to breaking. Additionally, the hydrophones possess isotropic compression characteristics and it is difficult for them to distinguish the direction of the source of an incoming signal. It would be much more desirable for piezoceramic hydrophones to exhibit anisotropic or directional characteristics. An additional disadvantage occurs because random pores formed on the surface of the ceramic material tend to absorb water, thereby further interfering with the effectiveness of the hydrophone.
Other piezoceramic devices would also benefit from anisotropic properties. For example, piezoelectric bender devices used in fans, motors and generators could have a higher degree of compliance in particular internal regions or directions to effect their desired bending operations. Certain benders have employed a metal core provided with holes. However, this introduces assembly and matching difficulties, e.g. the ceramic and metal exhibit different coefficients of expansion, thereby significantly reducing the advantages of this construction.
Other ceramic devices also suffer from disadvantages due to random porosity. Ceramics are generally weaker under tension than under compression and such materials typically exhibit very little deformation under stress. Accordingly, any imperfection such as a random pore in the ceramic material causes a stress concentration and stress enhancement which often results in cracking.
Random air-filled pores are used to provide heat insulation in refractory ceramics, such as are used in kilns. However, because such porosity weakens the ceramic structure the use of air-filled pores in refractory structures is limited.
There is a device, U.S. Pat. No. 3,829,356, which discloses very large planar voids between ceramic layers. However, only a single such void is provided between each ceramic layer and each void is immediately filled with conductive material to form internal electrodes so that the device can be used as a capacitor.